1. Field of the Invention
The present invention relates to an imaging apparatus for focusing a light onto an image to generate an electric signal representing thus focused light image with a broader dynamic range.
2. Description of the Prior Art
In FIG. 12, a conventional imaging apparatus is shown. The conventional imaging apparatus Ic includes an image pickup device 1, an analog signal processor 2, an analog to digital converter 3, and an image pickup device controller 4. The image pickup device focuses a light onto an object image to produce an electric signal based on thus obtained image. The imaging pickup device 1 accumulates the electric charge according to the quantity of light incident thereto for one imaging period. The imaging pickup device 1 outputs the accumulated electric charge as an image signal Sd representing the magnitude of thus accumulated electric charge, and further reset the amount of electric charge accumulated therein to zero.
In FIGS. 13 and 14, electric charge accumulation characteristics of the image pickup device 1 are shown. FIG. 13 shows the quantity of accumulated electric charge with respect to incidental period. FIG. 14 shows the quantity of accumulated electric charge with respect to the quantity of incidental light. In both of figures, "Q0" at the vertical axis indicates a saturation level which is a maximum accumulative electric charge of the image pickup device 1. Note that the amount Dp of incidental light Qinc at the time when the image pickup device reaches the saturation level Q0 represents a dynamic range of the image pickup device.
In FIG. 12, the analog signal processor 2 applies various process such as a noise filtration, a gamma correction, and amplification to the image signal Sd. The analog to digital converter 3 converts the electric signal processed by the analog signal processor 2 into a digital signal which will be transferred to the externals.
The image pickup device controller 4 has an electric charge accumulation controller 4a and an electric charge transmission controller 4b. The accumulation controller 4a determines the quantity of the electric charge which the image pickup device 1 accumulates therein. The transmission controller 4b controls the image pickup device 1 to transmit the electric charge accumulated therein as a signal Sd to the analog signal processor 2.
With reference to FIG. 15, the operation of the image pickup device controller 4 when the image pickup device is comprised of a photo diode and a vertical CCD (charge coupled device) is described. The accumulation controller 4a produces a charge accumulation control signal Sa indicating the quantity of the electric charge to be accumulated in the photo diode of the image pickup device 1.
The charge accumulation control signal Sa having pulses changing between a first level V0 and a second level V2 by a predetermined period. The first level V0 corresponds to the maximum accusative charge corresponding to the saturation level Q0. The second level V2 corresponds to "zero" charge level Q2. One period between each pulse of the signal Sa corresponds to one vertical blanking period of the image signal Sd.
The transmission controller 4b produces a first charge transfer signal Sb similar to the charge accumulation control signal Sa. The first charge transfer signal Sb has the same frequency as, but is delayed by a predetermined period Ta from the signal Sa. For this period Ta, the image pickup device 1 accumulates the electric charge therein. The transmission controller 4b further produces a second charge transfer signal Sc which is a combination of plural pulses of vertical transmission signal and horizontal transmission signal. Each batch of plural pulses corresponds to a signal vertical blanking period.
The photo diode is set to the saturation level Q0 and to zero level Q2 at the leading edge and trailing edge, respectively, of the charge accumulation control signal Sa. At the trailing edge of the signal Sa, the photo diode begins to accumulate the electric charge, and then transmits thus accumulated electric charge to the vertical CCD at the leading edge of the signal Sb. Specifically, during the accumulation period Ta between the pulses of signals Sa and Sb, the photo diode accumulates the electric charge and outputs. On receipt of each pulse of the second charge transfer signal Sc, the vertical CCD outputs the electric charge from the left top to the right bottom thereof, line by line, as the image signal Sd.
As apparent from the above, the image pickup device 1 can not accumulate or outputs the electric charge more than the saturation level Q0 even if the light is being incident thereto over this level Q0. In other words, when a light stronger than a light level that is determined according to a property of the image pickup device 1 is incident to the pickup device 1, it is impossible to express the strength of that incidental light as the quantity of electric charge accumulated in the image pickup device 1.
Therefore, according to the conventional imaging apparatus Ic, the digital signal output from the A/D converter 3 can not represent the strength of the light that has been incident to the image pickup device 1 beyond the saturation level Q0 of the image pickup device 1. In other words, it is impossible to reproduce an incidental image light whose brightness band is beyond a brightness range that is determined based on this saturated electric charge quantity. Thus, an imaging which can reproduce an incidental light with a broader dynamic range is needed.